A laser is a device that emits light or electromagnetic radiation with a high degree of spatial and temporal coherence through a process of optical amplification based on the stimulated emission of photons. The laser operation is known as Light Amplification by Stimulated Emission of Radiation. Spatial coherence typically is expressed through the output being a narrow beam which is diffraction-limited. For example, light beams emitted by a laser can be focused to very tiny spots, e.g., achieving a very high irradiance, or be focused into a beam of low divergence and concentrate power at a large distance.
Components of a laser include an energy supply, a gain medium to amplify light by stimulated emission, and an optical feedback mechanism. A laser amplifies light of a specific wavelength through the gain medium, thus increasing power. For the gain medium to amplify light, it needs to be supplied with energy, e.g., by an electrical current, or by another light at a different wavelength. An optical cavity (e.g., a pair of mirrors on either end of the gain medium) can serve as the optical feedback mechanism. For example, light can be directed back and forth between the mirrors, passing through the gain medium while being amplified each time. In some laser designs, one of the two mirrors of the optical cavity (the output coupler) is partially transparent to allow some light to escape through this mirror. The laser light emitted may spread out or form a narrow beam based on the design of the optical cavity.